Multiple Access in OFDM Systems

In one OFDM symbol, the data is modulated using Nc subcarriers and when there are multiple users, each user may be allocated a set of subcarriers, i.e., the data transmission to a particular user can take place using the allotted set of subcarriers. This is illustrated using the OFDM time and frequency grid shown in Fig. 3.5. Each rectangle in the grid can be considered as a resource and it can be allocated to any particular user in an OFDM symbol time. The user can employ a specific type of modulation depending on the quality of service (QoS) requirements and the channel quality allocated to him. Therefore, OFDM systems can offer flexibility in modulation order and multiple access. In addition, the base station must send the signaling information to each user indicating the subcarriers and time slots allotted to the user.

In general, it can be said that uncertainties concerning OFDM as a multiple access

concept concerns the system uplink. The main issue is to keep the mobile synchronized to the base station in time and frequency grid. This means that the mobiles must transmit the information with some timing advance due to the different propagation delay of the radio channels. The mobiles need to be synchronized to preserve the system orthogonality and avoid intercarrier interference (ICI). On the contrary, the downlink follows the same paths as the broadcast concepts and has already proven functional. All users are always orthogonal to each other because they are multiplexed in the base station. If a mobile loses the synchronization on the downlink, the only thing that happens is that the connection is lost. No other user will suffer from this type of failure. In the following, the OFDM transmission technique combining with multiple access techniques, such as FDMA and TDMA will be introduced.

3.3.1 Frequency Division Multiple Access (FDMA)

In OFDM-FDMA systems, each user is allocated a predetermined band of subcarriers and each user’s data is transmitted using only the subcarriers allotted to the user. This scheme can also be defined as OFDMA systems. In each allocated subcarrier, adaptive bit loading can be performed depending on the subchannel SNR to achieve the user’s requirements. By allocating distinct sets of subcarriers to different users, the available bandwidth can be flexibly shared between different users while avoiding any multiple access interferences (MAI) between them. There are variations possible in OFDM-FDMA systems, such as block FDMA and interleaved FDMA, which are stated below.

In the block FDMA scheme, each user is allocated a group of adjacent subcarriers as illustrated in Fig. 3.6. The different shades in Fig. 3.6 represent different users and the subcarriers allocated to them. The allocation of blocks to users can be accomplished by using the greedy algorithm. Assuming that each user is allocated only one block, the first block is allocated to the user with the best SNR of the block in the first step. Then, this particular user and the allotted block are removed from the search and the procedure is continued for the next block until all blocks are allocated. In most cases, the adjacent subcarrier gains are highly correlated and consequently, the bit loading of the block can be considered together. It means that the same modulation mode can be

used for all subcarriers in the block. The main advantage of the block FDMA scheme is easy allocation of subcarriers with less computational complexity but it lacks in robustness. There is a high probability that all subcarriers allocated to a user will fade at the same time. As a remedy, an improvement called interleaved FDMA will be introduced in the following.

In the interleaved FDMA scheme, the subcarriers assigned to a particular user are interlaced with other users’ subcarriers in the frequency domain. If a deep fade occurs, only a single subcarrier of a particular user is affected and the data can be recovered by using coding techniques across many OFDM symbols. Fig. 3.7 illustrates the interleaved FDMA concept.

The key advantages of OFDM-FDMA can be summarized as follows:

1. No multiple access interferences (MAI).

2. Incoherent or coherent modulation.

3. Adaptation to channel characteristics 4. Robustness against estimation errors.

3.3.2 Time Division Multiple Access (TDMA)

In OFDM-TDMA systems, each user is assigned a time slot during which all the subcarriers can be used for the particular user. This is illustrated in Fig. 3.8. The duration of each slot is assumed equal to the OFDM symbol. Due to the variations of the subcarrier channel gains for users, channel gains for some subcarriers may be quite low, while they are quite high for other subcarriers. Owing to the channel gain variations regarding one specific subcarrier for different users, the subcarrier with a low channel gain for one user may experience a high channel gain for some other users.

However, OFDM-TDMA doesn’t provide the flexibility to adapt fully to these channel gain variations.

The key advantages of OFDM-TDMA can be summarized as follows:

1. Incoherent or coherent modulation.

2. Adaptation to channel characteristics.

3. High coding gain introduced by the diversity.

4. Robustness against estimation errors.

5. No multiple access interferences (MAI) in case of synchronization errors 6. Easy implementation.

3.4 Dynamic Subcarrier Allocation Algorithms